Continuous casting of molten steel is carried out, for example, by pouring molten steel received from a ladle into a tundish, through a molten steel pouring nozzle secured to the bottom wall of the tundish, into a vertical mold arranged below the molten steel pouring nozzle, to form a cast steel strand, and continuously withdrawing the thus formed cast steel strand into a long strand.
As the above-mentioned molten steel pouring nozzle, a nozzle comprising an alumina-graphite refractory is widely used in general.
However, the molten steel pouring nozzle comprising an alumina-graphite refractory has the following problems:
When casting an aluminum-killed molten steel, aluminum added as a deoxidizer reacts with oxygen present in molten steel to produce non-metallic inclusions such as .alpha.-alumina. The thus produced non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, to clog up the bore, thus making it difficult to achieve stable casting. Furthermore, the non-metallic inclusions such as .alpha.-alumina, thus accumulated onto the surface of the bore peel off or fall down, and are entangled into the cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned reduction or clogging of the bore of the molten steel pouring nozzle caused by the non-metallic inclusions such as .alpha.-alumina present in molten steel, there is a popularly used method which comprises ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, to prevent the non-metallic inclusions such as .alpha.-alumina present in molten steel from adhering and accumulating onto the surface of the bore.
However, the above-mentioned method comprising ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, has the following problems:
A larger amount of the ejected inert gas causes entanglement of bubbles produced by the inert gas into the cast steel strand, resulting in the production of defects such as pinholes in a steel product after the completion of rolling. This problem is particularly serious in the casting of molten steel for a high-quality thin steel sheet. A smaller amount of the ejected inert gas causes, on the other hand, adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. In the casting of molten steel for a long period of time, a stable control of the amount of ejected inert gas from the surface of the bore of the molten steel pouring nozzle becomes gradually more difficult, according as a structure of the refractory forming the molten steel pouring nozzle degrades. As a result, the non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. Furthermore, in the casting of molten steel for a long period of time, a local erosion of the surface of the bore of the molten steel pouring nozzle is considerably accelerated by the ejected inert gas. This makes it impossible to continue the ejection of the inert gas and may cause rapid clogging of the bore.
With a view to preventing reduction or clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 62-148,076 dated July 2, 1987, a molten steel pouring nozzle formed of a refractory consisting essentially of:
(hereinafter referred to as the "prior art 1").
However, the above-mentioned molten steel pouring nozzle of the prior art 1 has the following problems:
Since the above-mentioned unstabilized zirconia and stabilized zirconia hardly react with non-metallic inclusions such as .alpha.-alumina, recesses are never formed on the surface of the bore of the molten steel pouring nozzle, through the reaction of the refractory forming the nozzle with the non-metallic inclusions such as .alpha.-alumina, and as a result, the non-metallic inclusions such as .alpha.-alumina never adhere onto the surface of the bore. However, when used for a long period of time at a high temperature within a range of from 900.degree.to 1,100.degree. C., unstabilized zirconia, which is a main constituent of the refractory forming the molten steel pouring nozzle, suffers from a transformation in the crystal structure thereof with an increased thermal expansion coefficient, resulting in the disintegration of crystal grains of unstabilized zirconia. In addition, a reduction reaction takes place between unstabilized zirconia having the disintegrated crystal grains and graphite, thus degrading the structure of the refractory.
Furthermore, when stabilized zirconia is used for a long period of time at a high temperature as described above, destabilization thereof is accelerated and transforms into unstabilized zirconia. This results in a phenomenon similar to that described above and the structure of the refractory is degraded.
As a result, recesses are formed on the surface of the bore of the molten steel pouring nozzle. Non-metallic inclusions such as .alpha.-alumina adhere and accumulate in these recesses, causing reduction or clogging of the bore of the molten steel pouring nozzle. It is thus difficult to use this molten steel pouring nozzle for a long period of time for continuously casting molten steel.
Furthermore, with a view to preventing reduction or clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 57-71,860 dated May 5, 1982, another molten steel pouring nozzle formed of a refractory consisting essentially of:
(hereinafter referred to as the "prior art 2").
However, the above-mentioned molten steel pouring nozzle of the prior art 2 has the following problems:
It is true that calcium oxide (CaO) rapidly reacts with non-metallic inclusions such as .alpha.-alumina, which are produced through the reaction of aluminum added as a deoxidizer with oxygen present in molten steel, to produce low-melting-point compounds. Therefore, calcium oxide has a function of preventing the non-metallic inclusions such as .alpha.-alumina from adhering and accumulating onto the surface of the bore of the nozzle. However, calcium oxide, when present alone, violently reacts with water or moisture in the air even at the room temperature to produce calcium hydroxide (Ca(OH).sub.2), which easily disintegrates and tends to become powdery, thus easily causing degradation of the structure of the molten steel pouring nozzle. Great care is therefore necessary for storing the molten steel pouring nozzle. In addition, because of the high thermal expansion coefficient of calcium oxide, a considerable thermal stress is produced in the interior of the molten steel pouring nozzle when calcium oxide is present alone and subjected to heating which causes a non-uniform temperature distribution, thus resulting in a lower thermal shock resistance of the molten steel pouring nozzle.
For the problems as described above, it is difficult to use a molten steel pouring nozzle made of a refractory, in which calcium oxide is present alone, for a long period of time for continuous casting of molten steel.
Finally, with a view to preventing reduction or clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 64-40,154 dated Feb. 10, 1989, further another molten steel pouring nozzle formed of a refractory consisting essentially of:
where, a content of calcium oxide in said calcium zirconate being within a range of from 23 to 36 weight parts relative to 100 weight parts of said calcium zirconate.
(hereinafter referred to as the "Prior art 3").
However, the above-mentioned molten steel pouring nozzle of the prior art 3 has the following problems:
For the purpose of overcoming the problems encountered in the prior art 2, in which calcium oxide is present alone, the molten steel pouring nozzle of the prior art 3 is formed of a refractory mainly comprising calcium zirconate. Therefore, it is true that contact of calcium oxide contained in calcium zirconate with the produced non-metallic inclusions such as .alpha.-alumina causes the acceleration of reaction between these components, thus producing low-melting-point compounds. Since calcium oxide is not present alone, no degradation of the structure of the molten steel pouring nozzle is caused. In the prior art 3, however, calcium oxide contained in calcium zirconate does not move sufficiently toward the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, so that calcium oxide does not come into sufficient contact with the produced non-metallic inclusions such as .alpha.-alumina. As a result, the production of low-melting-point compounds brought about by the reaction between calcium oxide and the non-metallic inclusions such as .alpha.-alumina is insufficient to effectively prevent adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle.
Under such circumstances, there is a strong demand for the development of a molten steel pouring nozzle which permits prevention of reduction or clogging of the bore of the nozzle and degradation of the structure of the refractory forming the nozzle economically and for long period of time without the use of a mechanical means such as the ejection of an inert gas, but such a molten steel pouring nozzle has not as yet been proposed.